Powder metal technology is well known to the persons skilled in the art and generally comprises the formation of metal powders which are compacted and then subjected to an elevated temperature so as to produce a sintered product.
Conventional sintering occurs at a maximum temperature of approximately up to 1,150.degree. C. Historically the upper temperature has been limited to this temperature by sintering equipment availability. Therefore copper and nickel have traditionally been used as alloying additions when sintering has been conducted at conventional temperatures of up to 1,150.degree. C., as their oxides are easily reduced at these temperatures in a generated atmosphere, of relatively high dew point containing CO, CO.sub.2 and H.sub.2. The use of copper and nickel as an alloying material is expensive. Moreover, copper when utilized in combination with carbon as an alloying material and sintered at high temperatures causes dimensional instability and accordingly the use of same in a high temperature sintering process results in a more difficult process to control the dimensional characteristics of the desired product.
Manufacturers of metal powders utilized in powder metal technology produce prealloyed iron powders which are generally more difficult to compact into complex shapes, particularly at higher densities (&gt;7.0 g/cc). Manganese and chromium can be incorporated into prealloyed powders provided special manufacturing precautions are taken to minimize the oxygen content, for example, by oil atomization. Notwithstanding this, these powders still have poor compressibilities compared to admixed powders.
Conventional means to increase the strength of powder metal articles use up to 8% nickel, 4% copper and 1.5% molybdenum, in prealloyed, partially prealloyed, or admixed powders. Furthermore double press double sintering can be used for high performance parts as a means of increasing part density. Conventional elements are expensive and relatively ineffective for generating mechanical properties equivalent to wrought steel products, which commonly use the more effective strengthening alloying elements manganese and chromium.
Moreover, conventional technology as disclosed in U.S. Pat. No. 2,402,120 teach pulverizing material such as mill scale to a very fine sized powder, and thereafter reducing the mill scale powder to iron powder without melting it.
Furthermore, U.S. Pat. No. 2,289,569 relates generally to powder metallurgy and more particularly to a low melting point alloy powder and to the usage of the low melting point alloy powders in the formation of sintered articles.
Yet another process is disclosed in U.S. Pat. No. 2,027,763 which relates to a process of making sintered hard metal and consists essentially of steps connected with the process in the production of hard metal. In particular, U.S. Pat. No. 2,027,763 relates to a process of making sintered hard metal which comprises producing a spray of dry, finely powdered mixture of fusible metals and a readily fusible auxiliary metal under high pressure producing a spray of adhesive agent customary for binding hard metals under high stress, and so directing the sprays that the spray of metallic powder and the spray of adhesive liquid will meet on their way to the molds, or within the latter, whereby the mold will become filled with a compact moist mass of metallic powder and finally completing the hard metallic particle thus formed by sintering.
U.S. Pat. No. No. 4,707,332 teaches a process for manufacturing structural parts from intermetallic phases capable of sintering by means of special additives which serve at the same time as sintering assists and increase the ductility of the finished structural product.
Finally, U.S. Pat. No. 4,464,206 relates to a wrought powder metal process for pre-alloyed powder. In particular, U.S. Pat. No. 4,464,206 teaches a process comprising the steps of communinuting substantially non-compactible pre-alloyed metal powders so as to flatten the particles thereof heating the communinuted particles of metal powder at an elevated temperature, with the particles adhering and forming a mass during heating, crushing the mass of metal powder, compacting the crushed mass of metal powder, sintering the metal powder and hot working the metal powder into a wrought product.
The processes as described in the prior art above present a relatively less cost effective process to achieve the desired mechanical properties of the sintered product.
It is an object of this invention to provide an improved process for producing sintered articles having improved dynamic strength characteristics and an accurate method to control same.
It is an aspect of this invention to provide a process of forming a sintered article of powder metal comprising blending carbon, and ferro alloy powder and lubricant with compressible elemental iron powder, pressing the blended mixture to form the article, and then high temperature sintering the article in a reducing atmosphere or under a vacuum.
Another aspect of this invention resides in a process of forming a sintered article of powder metal comprising blending carbon and ferro alloy powder and lubricant with compressible elemental iron powder, pressing the blended mixture to form the article and then high temperature sintering the article in a neutral or reducing atmosphere with a dew point of not higher than -20.degree. C. or under a vacuum to produce sintered parts which contain typically not more than 250 ppm oxygen. Moreover it is another aspect of this invention that articles are brought to a temperature not greater than 150.degree. C. after sintering in a low dew point atmosphere of not higher than -30.degree. C.
It is another aspect of this invention to provide a process of forming a sintered article of powder metal comprising; selecting elemental iron powder, determining the desired properties of said sintered article and selecting, a quantity of carbon, and a combination of ferro alloy powder from the group of ferro manganese, ferro chromium, ferro molybdenum, ferro vanadium, ferro silicon and ferro boron and selecting the quantity of same; grinding separately each said ferro alloy to a mean particle size of approximately 8 to 12 microns and substantially all of said ferro alloy having a particle size of less than 25 microns; introducing a lubricant while blending the carbon, and ferro alloy, with said elemental iron powder; pressing the mixture to form the article; and then high temperature sintering the article at a temperature between 1,250.degree. C. and 1,350 .degree. C. in a neutral atmosphere or a reducing atmosphere such as 90% nitrogen and 10% hydrogen, so as to produce the sintered article of powdered metal.
It is another aspect of this invention to provide an as-sintered ferrous metal product comprising a compacted and sintered mass composed of a blend of elemental iron, carbon and ferro manganese alloy having a mean particle size of approximately 8 to 12 microns, subjected to a high temperature sinter so as to result in an as-sintered mass having between 0.5 to 2.0% manganese and between 0.2 to 0.85% carbon composition wherein said product is machined or coined to final dimensional requirements.
It is another aspect of this invention to provide a gas quenched ferrous metal product comprising of a blend of elemental iron, carbon, ferro manganese, ferro chromium and ferro molybdenum having a mean particle size of approximately 8 to 12 microns, subjected to a high temperature sinter and then gas pressure quenching said product at a pressure of for example up to 5 bar so as to result in a hardened sintered mass having between 0.5 to 2.0% manganese, between 0.5 to 1.5% molybdenum between 0 to 1.0% chromium and between 0 to 0.6% carbon composition.
It is another aspect of this invention to provide a sinter-hardened ferrous metal product comprising a compacted and sintered mass composed of a blend of elemental iron, carbon, and ferro manganese alloy and ferro molybdenum alloy, said ferro manganese and ferro molybdenum alloy having a mean particle size of approximately 8 to 12 microns, subjected to a high temperature sinter so as to result in a sinter hardening mass having a up to 1.0 to 2.0% manganese, between 0 to 1.0% molybdenum, and between 0.5 to 0.85% carbon composition. It has been found that sinter-hardening produces an article which hardens to a hardness greater than HRB 90 in the furnace cooling zone.
It is another aspect of this invention to provide a high strength ferrous metal product comprising compacted and sintered mass composed of a blend of elemental iron powder, carbon, ferro manganese alloy, ferro chromium and ferro molybdenum having a mean particle size of approximately 8 to 12 microns, subjected to a high temperature sinter which is induction hardened and air cooled to impart impact strength, having between 0.5% to 2.0% manganese, between 0.5 to 2.0% chromium, between 0 to 1.0% molybdenum and between 0.1% to 0.6% carbon.
It is another aspect of this invention to provide a high ductility ferrous metal product comprising a compacted and sintered mass composed of a blend of elemental iron powder, carbon, ferro chromium and ferro molybdenum alloy having a mean particle size of approximately 8 to 12 microns, subjected to a high temperature sinter which is induction hardened and cooled in a neutral or reducing atmosphere to impart impact strength so as to result in a mass having between 0.5 to 2.0% chromium, between 0 to 1.0% molybdenium and between 0.1 to 0.6% carbon composition.
It is another aspect of this invention to provide a high ductility ferrous metal product comprising a compacted and sintered mass composed of a blend of elemental iron, carbon, chromium and molybdenum, the ferro alloys having a mean particle size of approximately 8 to 12 microns and subjected to a high temperature sinter. This alloy may be used for further deformation to final dimensional requirements by extrusion, rolling and forging.